During certain phosphor synthesis and lamp fabrication steps, the finely divided luminescent materials may be exposed to oxidizing (oxygen-rich) atmospheres at elevated temperatures. An example of this is the so-called `lehring` process used to burn away organic aqueous lamp coating dispersion. It is well known that the brightness of the finished fluorescent lamp may be reduced significantly as a result of the lehring operation (the so-called `lehrloss` ). This reduction in brightness may result from a partial oxidation of reactive low valence ions present in the phosphor lattice.
A somewhat more involved example relates to the lamp and process described in above-referenced U.S. Pat. Nos. 4,585,673 and 4,710,674, disclosing the formation of protective coatings (typically alumina coatings) upon the surfaces of finely divided phosphor particles via chemical vapor deposition using an organometallic precursor in a gas-fluidized bed. When calcium halophosphate or manganese-doped zinc silicate is alumina-coated via the process described in the '673 patent, and when fluorescent lamps are fabricated from the coated phosphor produced therefrom, these lamps display much better lumen maintenance than do similar lamps fabricated using the virgin (uncoated) zinc silicate phosphor.
During the fabrication of such lamps, the phosphor particles are typically dispersed in an aqueous medium. Unfortunately, if the water-based zinc silicate suspension is held-over for several days before use (a typical situation), the beneficial effects associated with the '673 can be lost. This `holdover` problem can be overcome, however, by annealing the alumina-coated phosphor in the air at a temperature between about 700.degree. C. and about 850.degree. C. for a period of time ranging from about 15 minutes to about 20 hours as described in U.S. Pat. No. 4,803,400 to Peters et al. Unfortunately, while this coated phosphor annealing process solves the holdover problem, it can also cause the zinc silicate phosphor to react with the alumina coating. Zinc and manganese can diffuse into the alumina coating, probably forming a mixture of zinc and manganese aluminates. The coated a `body color`, and can suffer a reduction in visible light emission upon exposure to an ultraviolet light source. Moreover, very similar phenomena are observed, as well, when the virgin (uncoated) phosphor is subjected to the annealing process. The increased body color and reduced brightness which result from annealing both the uncoated and the '673 coated zinc silicate phosphor are believed to result partially from the oxidation of some of the divalent manganese ions located on the surface of the uncoated phosphor particles or within and on the surface of the reactive alumina coating.
Prior to the present invention, there was no known means of preventing these detrimental interactions between the phosphor and the oxygen-rich atmosphere within the annealing furnace. By means of the method described below, these detrimental interactions are virtually eliminated, allowing a phosphor coated by the process disclosed in the '673 patent to be thoroughly annealed without suffering reflectance or brightness losses.
Also described herein are lamps in which high brightness, high maintenance zinc silicate phosphors are used as components of triblend phosphors. As discussed previously, coated zinc silicate phosphors are unstable in the water based suspension systems used to manufacture fluorescent lamps. The phosphors must be annealed to stabilize the coated phosphor. However, the performance of such a phosphor suffers in terms of both brightness output and lumen maintenance after annealing. Attempts to improve the base phosphor performance prior to annealing include remilling and refiring (RMF) the phosphor. Such an alumina coated "RMF" phosphor shows improved lumen characteristics when used as a component in the high color rendition triblend layer. However, the remilling and refiring process results in a large loss of starting material, thus increasing cost of the phosphor. The present invention solves these problems in a novel and economical way.
In a related application, an improved compact fluorescent lamp can be manufactured using a coated phosphor as the green-emitting component. Compact fluorescent lamps of the twin-tube and double twin-tube variety have become important for energy conservation in recent years since they have efficiencies which far exceed those of conventional incandescent lamps. While these lamps are very cost-effective with very short payback periods, they, nevertheless, have high initial costs which have limited the scope of applications in which they have been exploited. Therefore, it is desirable to further reduce the cost of these lamps through the use of less expensive non-rare-earth-containing substitutes.
The compact fluorescent lamps currently employ two rare-earth based phosphors. They are Y.sub.2 O.sub.3 :Eu (Sylvania Type 2342) for the red emission and Mg aluminate: Ce,Tb (Sylvania Type 2293) for the green emission. No blue-emitting phosphor is required, since the blue components of the mercury discharge are used to achieve the proper color temperature of the emitted `white` light. More recently, LaPO.sub.4 :Ce,Tb, manufactured by Nichia Corporation, is being considered as a replacement for the Type 2293. Because these materials contain expensive rare-earths as the activators, they are some of the most expensive phosphors commercially used.
As mentioned previously, a green-emitting zinc orthosilicate phosphor activated with manganese, also known by the mineral name willemite can be improved by the application of a bilayer coating prior to annealing. The bilayer consists of a thin coating of silica applied between the base phosphor and a conformal alumina coating which is exposed to the mercury discharge. The base phosphor is a zinc silicate phosphor doped with manganese and tungsten as described in U.S. patent application Ser. No. 06/902,265, now abandoned. This phosphor can be manufactured on production scale equipment using a single step firing procedure which provide very high yields (typically 90%). These high yields and efficiencies of scale provide substantial phosphor cost savings which far outweigh the cost of applying the intermediate silica layer. The use of the willemite phosphor which has been coated with silica and then with alumina can be used as the green-emitting component of economical, high color rendition fluorescent lamps.
The properties of Cool-White alkaline earth halophosphate phosphors, i.e., those which fall within class "C" of the Geldart Classification Scale, are apparently affected differently by an alumina coating than those of the zinc silicate phosphors described above. In the alumina coated zinc silicate phosphors, the lumen maintenance is very high in lamp tests. In the alumina coated Cool-White phosphor, the lumen maintenance is significantly improved. However some loss of brightness over time is still observed, the loss increasing with the lamp loading. The lumen maintenance decrease is particularly noticeable in the high load (HO) and very high load (VHO) lamps. The lumen maintenance can be further improved by activation with cadmium; however, the trend in recent years has been away from the use of heavy metals, prompting a search for ways to raise the lumen maintenance without the use of such elements. The lumen maintenance of an alkaline earth halophosphate phosphor, for example a calcium halophosphate activated with antimony and manganese can be improved by the application, prior to annealing, of the bilayer coating described herein. The bilayer consists of a thin coating of silica applied between the base phosphor and a conformal alumina coating which is exposed to the discharge. The calcium halophosphate phosphor which has been coated with silica and then with alumina can be used to great advantage in high load and high color rendition fluorescent lamps.